This invention is generally directed to processes for the preparation of toner compositions, and more specifically, to semisuspension polymerization processes for the preparation of black and colored, like cyan, yellow, magenta, and the like, toners. In one embodiment, the present invention is directed to semisuspension polymerization processes for the economical preparation of toners with, for example, an average volume diameter of from about 3 to about 25, and preferably from about 3 to about 7 microns, wherein a mixture of monomer or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component are bulk polymerized until partial polymerization to the onset of the gel-effect is accomplished; followed by adding to the formed partially polymerized polymer pigments or dyes, optional additives such as charge control materials, low molecular weight waxes, such as polypropylene, or polyethylene, and the like to form an organic phase, and then mixing this organic phase with, for example, a high shear mixer to obtain a homogeneous organic mixture; subsequently dispersing the resulting organic mixture in water containing a stabilizing component, which dispersing can be accomplished, for example, by a high shear mixer; transferring the resulting suspension to a reactor; and effecting polymerization thereof; followed by cooling, optional washing, and drying. The toner composition obtained can be optionally blended with surface additives, which may function as flow aids, such as colloidal silicas and the like. The gel-effect is a known phenomenon in the polymerization process and the onset of the gel-effect can be indicated as a conversion after which both the rate of polymerization and molecular weight of polymer produced during polymerization increase rapidly. Conversion is determined gravimetrically. The degree of conversion at which the gel-effect commences is related to the volume of polymer, molecular weight of polymer and the specific monomer system. This conversion x.sub.b can be represented by x.sub.b =K.sub.c d.sub.p /dM.sub.n.sup.0.5 where K.sub.c is an entanglement parameter characteristic of the particular system, d.sub.p is the density of the pure polymer produced during polymerization, d is the overall density of the monomer/polymer mixture and M.sub.n is the number average molecular weight of polymer produced during polymerization. Conversion at the onset of the gel-effect (x.sub.b) for a given polymerization system can be calculated using this equation. In the absence of a gel-effect, the conversion, x, at any time, t, during the polymerization can be represented by x=1-exp(-kt) where x is the fractional conversion of monomer to polymer, t is the reaction time and k is a reaction rate constant. The onset of the gel-effect can also be determined as that time of reaction at which the actual conversion exceeds the conversion predicted by the equation x=1-exp(-kt) by a factor of 1.2, that is the actual conversion exceeds the conversion that would be obtained in the absence of gel-effect by 20 percent. In one embodiment, the process of the present invention comprises the bulk polymerization of comonomers, such as styrene methacrylates, like styrene-n-butylmethacrylate or styrene acrylates, like styrene butyl acrylate, and the like suitable for toner resins with an initiator/catalyst up to a conversion of the comonomers to polymer which is within from about 1 to about 5 percent of the conversion of the onset of the gel effect; terminating the aforementioned bulk polymerization by cooling the partially polymerized monomer or comonomers; adding pigments, and other optional additives, such as charge additives followed by mixing with, for example, a high shear mixer to form the organic phase; dispersing this organic phase into the aqueous phase comprised of an aqueous solution of a suspension stabilizer and optionally an aqueous phase inhibitor such as potassium iodide; forming a suspension with, for example, a high shear homogenizer with particles with an average volume particle diameter of from about 3 to about 7 microns; heating to initiate suspension polymerization, and retaining the mixture at a high temperature of from about 50 to about (about as used herein includes points in between the parameters recited) 120.degree. C. and more preferably from between about 60 to about 80.degree. C., thereby completing the conversion of comonomers to polymer, separating the polymer product by filtration after cooling; and subsequently washing to eliminate the stabilizer; followed by filtration, and drying, for example, by freeze drying, vacuum drying, spray drying or fluid bed drying whereby there results polymer particles with an average particle diameter of from about 3 to about 7 microns.
Advantages associated with the processes of the present invention in embodiments include improved pigment dispersion, substantial avoidance of coalescence, narrow particle size distribution, for example, from about 1.1 to 1.3, the preparation of small, for example in embodiments 3 to 7 microns diameter, black and color toners with high projection efficiency and high gloss, and toner particles with less residual surfactant on the surface and therefore better flow.
Toners have been prepared generally by fusion mixing of pigments (colorants), charge control agents and other additives into thermoplastic resins to disperse them uniformly therein. In view of the high viscosity of the mixture, a considerable amount of energy is needed to achieve uniform dispersion of pigments and other additives in the toner resin. The mixture is then cooled, followed by pulverization and classification into desired particle sizes and particle size distribution. It is known that pulverization is an energy intensive step in this process. This preparation method is capable of producing excellent toners, but requires the use of several steps which are costly, energy intensive and are limited in certain respects. In the process for producing toners by pulverization, the material most usually be fragile so as to be readily pulverized to a certain extent. Therefore, some thermoplastic resins which are not fragile but have good fusing performance are not usually selected for the aforementioned prior art processes. Also, if the material is too fragile, it may be excessively micropulverized and, therefore, the fines portion of the particles must be uneconomically removed. These limitations become increasingly severe for smaller particle size toners. Moreover, when a material with a low melting point is employed to improve fusing performance of the toner, fusion of such material may occur in the pulverizing device or the classifier.
Accordingly, in order to avoid or minimize the disadvantages of the pulverization method, there have been proposed processes for producing toner wherein the toner particles were formed and correct particle size distribution produced in a reactor. These processes include dispersion polymerization, suspension polymerization, emulsion polymerization, and the like. Disclosed in U.S. Pat. No. 4,486,559 is the preparation of a toner composition by the incorporation of a prepolymer into a monomer/pigment mixture, followed by emulsion polymerization, see for example columns 4, 5 and 8 of this patent. Also, methods of preparing toner, including suspension/dispersion polymerization, are detailed in columns 1 and 2 of this patent. In these processes, the pigment and additives such as charge control components are added to a monomer or comonomers prior to polymerization. Particle formation is achieved by the dispersion of the pigmented monomer or comonomers in a continuous phase, such as water, and the droplets of pigmented monomers are then polymerized to form toner particles. One advantage of these processes as compared to many other methods is the elimination of fusion mixing (Banbury/extruder) and pulverization classification processing. Nevertheless, it can be difficult with these processes to accomplish polymerization of pigmented monomer droplets in a diameter range of 3 to 25 microns with a narrow distribution of particle diameter of, for example, 1.3. Also, suspension failure is common with these processes especially when monomer droplet diameter is less than 50 microns and as polymerization proceeds in the sticky region (10 to 60 percent conversion). Further, it is difficult to conduct the polymerization of pigmented monomer droplets since, for example, it is known that polymerization of free radical type monomers are hindered, and many times prevented by the presence of various pigments, especially carbon black. Another disadvantage of some of the prior art processes for the preparation of toners resides in the resulting poor dispersion of the pigment and other additives within particles which is believed to be caused by the lack of mixing within individual particles during polymerization and incompatibility of pigment with polymer produced during polymerization which results in pigment aggregation. Moreover, some of the prior art processes for the preparation of particles can be costly, or not as economical as desired. These and other disadvantages are avoided or minimized with the semisuspension polymerized toner processes of the present invention.
Also mentioned are U.S. Pat. Nos. 4,486,559, which discloses the incorporation of a prepolymer into a monomer toner mix followed by emulsion polymerization; 4,680,200 and 4,702,988, which illustrate emulsion polymerization; and 4,797,339 and 4,996,127 which disclose a process in which small primary particles are produced by emulsion polymerization, and then these particles are imbedded with pigment on the surface and aggregated, thereby providing improved pigment dispersion, however, this process involves many steps, thereby rendering it costly and reducing yields. In addition, control of the molecular weight and molecular weight distribution is difficult because of the very low molecular weights required and the natural tendency of emulsion polymerizations to produce very high molecular weights. Furthermore, because the pigment is on the surface of the aggregated particle, charging behavior of the toner can vary when different pigments are used.
In patentability search reports, there were recited as background U.S. patents disclosing suspension polymerization U.S. Pat. Nos. 4,077,804; 4,601,968; 4,626,489; 4,816,366 and 4,845,007; 5,043,404 directed to semisuspension polymerization; and 3,954,898, which discloses bulk and suspension polymerization, see the Abstract for example.
In U.S. Pat. Nos. 5,164,282, the disclosure of which is totally incorporated herein by reference, there are illustrated processes for the preparation of toners, and more specifically, semisuspension polymerized toner processes in which a mixture of monomer or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component is bulk polymerized until partial polymerization, that is for example from about 10 to about 40 percent of monomer or comonomers is converted to a polymer; thereafter mixing the partially polymerized product with pigments, optional charge control agents and other additives with, for example, a high shear homogenizer to form a uniform organic phase, dispersing the organic phase in water containing a stabilizing component with, for example, a high shear mixer to produce a narrow particle size toner suspension; and polymerizing the suspension product. The toner obtained can then be washed/dried and dry blended with surface flow aid additives. However, the processes described in U.S. Pat. No. 5,164,282 do not provide as good a dispersion as the process of the present invention since, for example, it does not accomplish, for example, the conversion at which the bulk polymerization must be terminated. In U.S. Pat. No. 5,164,282, it is indicated that the bulk polymerization should be conducted until partial conversion from 10 to 40 percent is achieved, and this will not usually provide uniform dispersion of pigment throughout the particle interior as individual pigment particles as determined by transmission electron microscopy, but rather will result in pigment particles migrating to the toner particle surface and/or aggregating in clusters of primary pigment particles. In order to achieve uniform dispersion of individual pigment particles throughout the particle interior and control of micromixing, it has been found that the partial polymerization should be conducted to within 1 to 5 percent of the onset of the gel-effect, which depends on the temperature and molecular weight of the polymer. Subsequently, when the suspension polymerization begins after the organic phase is dispersed in the aqueous phase, the polymerization rate, molecular weight and viscosity in the particle will increase rapidly, thereby restricting diffusion of pigment and other additives within the toner particles and ensuring good pigment dispersion. Pigment dispersion is influenced primarily by the initial polymer viscosity, which depends on the type of polymer, molecular weight of polymer, polymer concentration and temperature, versus time profile in the polymerizing particle and the size of the pigment particle. For a given pigment diameter, pigment diffusion can be minimized by increasing the initial viscosity of the organic phase and by reducing the time available for diffusion to occur, which is the time between when the dispersion is created and when the viscosity in the particles reaches a very high value, typically when the conversion is greater than 80 percent, and by maximizing the viscosity during that period initial viscosity of the organic phase can be increased using semisuspension polymerization by increasing polymer concentration and/or the molecular weight of the polymer produced during the polymerization or by reducing the temperature. Reducing the time available for diffusion to occur can be achieved by conducting the suspension polymerization in the gel-effect regime so that the rate of polymerization is very high, for example in the range of 0.05 to 1 moles/liter/minute. Therefore, these objectives can be achieved by conducting the bulk polymerization to within 1 to 5 percent of the onset of the gel-effect. When the bulk polymerization is terminated before the onset of the gel-effect, the rate of viscosity increases and increasing the conversion in the polymerizing toner particles will be more gradual than when the bulk polymerization was terminated at the onset of the gel-effect since the subsequent rate of increase of polymer concentration will be less, and the viscosity is very sensitive to increases in conversion (polymer concentration). This lower rate of increase in viscosity will result in uncontrolled diffusion of pigment to the particle surface and to the formation of clusters of primary pigment particles and therefore overall poor pigment dispersion. If the bulk polymerization is conducted beyond the onset of the gel-effect, the reaction will be very difficult to control because of the rapid increase in the viscosity which results in a wide GSD of the particles. Therefore, by terminating the bulk polymerization within 1 to 5 percent of the onset of the gel-effect, micromixing is controlled and pigment dispersion is improved, yielding toner with high projection efficiency and high gloss in embodiments of the present invention.